Testing the axion-conversion hypothesis of 3.5 keV emission with polarization

The recently measured 3.5 keV line in a number of galaxy clusters, the Andromeda galaxy (M31) and the Milky Way (MW) center can be well accounted for by a scenario in which dark matter decays to axion-like particles (ALPs) and subsequently convert to 3.5 keV photons in magnetic fields of galaxy clusters or galaxies. We propose to test this hypothesis by performing X-ray polarization measurements. Since ALPs can only couple to photons with polarization orientation parallel to magnetic field, we can confirm or reject this model by measuring the polarization of 3.5 keV line and comparing it to the orientation of magnetic field. We discuss luminosity and polarization measurements for both galaxy cluster and spiral galaxy, and provide a general relation between polarization and galaxy inclination angle. This effect is marginally detectable with X-ray polarimetry detectors currently under development, such as the enhanced X-ray Timing and Polarization (eXTP), the Imaging X-ray Polarimetry Explorer (IXPE) and the X-ray Imaging Polarimetry Explorer (XIPE). The sensitivity can be further improved in the future with detectors of larger effective area or better energy resolutions.